The gene for the photoprotein aequorin has been incorporated into several mammalian cell lines. Because the luminescence of this protein reflects the local Ca++ concentration, it is very useful for assaying cellular Ca++ signals that are activated or blocked by drugs of potential therapeutic value. A major experimental difficulty lies in the very low level of light that is emitted, since only one photon is liberated by each molecule of aequorin, and we have calculated that there are only 20,000 molecules per cell. We plan to modify a photon-counting instrument that we built to measure luminescence from frog oocytes. The photon count will be used as an assay to determine the activity of drugs that influence cell Ca++ metabolism. Large numbers of chemicals will be screened for activity, so the instrument will be automated as much as possible. We have the difficult job of injecting small quantities (20 to 50 microliters) of fluids and cell suspensions.